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Abstract

Feeding is dynamically regulated by the palatability of the food source and the physiological needs of the animal. How consumption is controlled by external sensory cues and internal metabolic state remains under intense investigation. Here, we identify four GABAergic interneurons in the Drosophila brain that establish a central feeding threshold which is required to inhibit consumption. Inactivation of these cells results in indiscriminate and excessive intake of all compounds, independent of taste quality or nutritional state. Conversely, acute activation of these neurons suppresses consumption of water and nutrients. The output from these neurons is required to gate activity in motor neurons that control meal initiation and consumption. Thus, our study reveals a layer of inhibitory control in feeding circuits that is required to suppress a latent state of unrestricted and nonselective consumption.

Abstract

The decision to engage in one behavior often precludes the selection of others, suggesting cross-inhibition between incompatible behaviors. For example, the likelihood to initiate feeding might be influenced by an animal's commitment to other behaviors. Here, we examine the modulation of feeding behavior in the fruit fly, Drosophila melanogaster, and identify a pair of interneurons in the ventral nerve cord that is activated by stimulation of mechanosensory neurons and inhibits feeding initiation, suggesting that these neurons suppress feeding while the fly is walking. Conversely, inhibiting activity in these neurons promotes feeding initiation and inhibits locomotion. These studies demonstrate the mutual exclusivity between locomotion and feeding initiation in the fly, isolate interneurons that influence this behavioral choice, and provide a framework for studying the neural basis for behavioral exclusivity in Drosophila.

Abstract

For an animal to survive in a constantly changing environment, its behavior must be shaped by the complex milieu of sensory stimuli it detects, its previous experience, and its internal state. Although taste behaviors in the fly are relatively simple, with sugars eliciting acceptance behavior and bitter compounds avoidance, these behaviors are also plastic and are modified by intrinsic and extrinsic cues, such as hunger and sensory stimuli. Here, we show that dopamine modulates a simple taste behavior, proboscis extension to sucrose. Conditional silencing of dopaminergic neurons reduces proboscis extension probability, and increased activation of dopaminergic neurons increases extension to sucrose, but not to bitter compounds or water. One dopaminergic neuron with extensive branching in the primary taste relay, the subesophageal ganglion, triggers proboscis extension, and its activity is altered by satiety state. These studies demonstrate the marked specificity of dopamine signaling and provide a foundation to examine neural mechanisms of feeding modulation in the fly.

Abstract

In insects, increasing evidence suggests that small secreted pheromone binding proteins (PBPs) and odorant binding proteins (OBPs) are important for normal olfactory detection of airborne pheromones and odorants far from their source. In contrast, it is unknown whether extracellular ligand binding proteins participate in perception of less volatile chemicals, including many pheromones, that are detected by direct contact with chemosensory organs. CheB42a, a small Drosophila melanogaster protein unrelated to known PBPs or OBPs, is expressed and likely secreted in only a small subset of gustatory sensilla on males' front legs, the site of gustatory perception of contact pheromones. Here we show that CheB42a is expressed specifically in the sheath cells surrounding the taste neurons expressing Gr68a, a putative gustatory pheromone receptor for female cuticular hydrocarbons that stimulate male courtship. Surprisingly, however, CheB42a mutant males attempt to copulate with females earlier and more frequently than control males. Furthermore, CheB42a mutant males also attempt to copulate more frequently with other males that secrete female-specific cuticular hydrocarbon pheromones, but not with females lacking cuticular hydrocarbons. Together, these data indicate that CheB42a is required for a normal gustatory response to female cuticular hydrocarbon pheromones that modulate male courtship.

A Drosophila DEG/ENaC channel subunit is required for male response to female pheromonesPROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICALin, H. P., Mann, K. J., Starostina, E., Kinser, R. D., Pikielny, C. W.2005; 102 (36): 12831-12836

Abstract

Odorants and pheromones as well as sweet- and bitter-tasting small molecules are perceived through activation of G protein-coupled chemosensory receptors. In contrast, gustatory detection of salty and sour tastes may involve direct gating of sodium channels of the DEG/ENaC family by sodium and hydrogen ions, respectively. We have found that ppk25, a Drosophila melanogaster gene encoding a DEG/ENaC channel subunit, is expressed at highest levels in the male appendages responsible for gustatory and olfactory detection of female pheromones: the legs, wings, and antennae. Mutations in the ppk25 gene reduce or even abolish male courtship response to females in the dark, conditions under which detection of female pheromones is an essential courtship-activating sensory input. In contrast, the same mutations have no effect on other behaviors tested. Importantly, ppk25 mutant males that show no response to females in the dark execute all of the normal steps of courtship behavior in the presence of visible light, suggesting that ppk25 is required for activation of courtship behavior by chemosensory perception of female pheromones. Finally, a ppk25 mutant allele predicted to encode a truncated protein has dominant-negative properties, suggesting that the normal Ppk25 protein acts as part of a multiprotein complex. Together, these results indicate that ppk25 is necessary for response to female pheromones by D. melanogaster males, and suggest that members of the DEG/ENaC family of genes play a wider role in chemical senses than previously suspected.

Abstract

Control of nitric oxide (NO) synthesis in the constitutive nitric-oxide synthases (NOS) by calcium/calmodulin is exerted through the regulation of electron transfer from NADPH through the reductase domains. This process has been shown previously to involve the calmodulin binding site, the autoinhibitory insertion in the FMN binding domain, and the C-terminal tail. Smaller sequence elements also appear to correlate with control. Although some of these elements appear well positioned to function in control, they are poorly conserved; their role in control is neither well established nor defined by available information. In this study mutations have been induced in the small insertion of the hinge subdomain, which has been shown recently to form a beta hairpin in structural studies of the neuronal NOS reductase domains adjacent to the calmodulin site and the autoinhibitory element. Modification of the small insertion in neuronal NOS tends to increase cytochrome c reduction but not NO synthetic activity; some modifications or deletions in the corresponding region in endothelial NOS modestly increase activity under some conditions. Unexpectedly, some minor changes in the sequence introduce a loss in the content of heme relative to flavin cofactors. Taken together, these results suggest that the small insertion protects the calmodulin binding site and that it may be a modulator of NOS activity.